The Kennedy’s motorcade drives through downtown Dallas on Nov. 22, 1963, moments before the shooting of President John F. Kennedy. (Bettmann/Corbis)

This is a guest post by University of Miami political scientists Joseph Uscinski and Joseph Parent. This article is based on portions of their forthcoming book “American Conspiracy Theories” (Oxford University Press, 2014).

Actually, there is not much truth in any of the above. Journalists have been quick to proclaim a “new age of conspiracy theories.” The only problem is that “new age” is typically just a synonym for “now.” For example, see 2011, 2010, 2004, 1994, 1991 and 1964. Fortunately, we have a much better sense of where conspiracy theories come from and why so many people believe them.

Conspiracy theories ignite when motive meets opportunity. For reasons we mostly attribute to socialization, some individuals tend to see the world more through a conspiratorial lens than others. We can think of this predisposition as a strong bias against powerful disliked actors that is not caused by partisanship, stupidity or psychopathology. In fact, people disposed to see conspiracies are just as likely to be Democrats as Republicans, and appear just as likely to be lauded (e.g. Thomas Jefferson) as reviled (e.g. Joseph McCarthy).

Once socialization stacks the tinder, political opportunity sets off the sparks. This means that the predisposition to see conspiracies must harmonize with political ideology and partisanship. Different conspiracy theories will therefore appeal to different people. Normally, when possibly incriminating information comes out, at best half of the political spectrum has an incentive to use it. And, depending on the quality of that information, about half of that half has the predispositions to try.

Because of this, most conspiracy theories will only attract a quarter to a third of the population. The Truther and Birther theories are good examples. (Respectively, these are the theories that the Bush administration either directed or permitted the 9/11 attacks, and that Barack Obama was foreign-born and faked his Hawaiian birth certificate.) They resonate with about 25 percent of the population, largely those with conspiratorial worldviews whose party stands to gain from the accusations. Polls show there is a great deal of symmetry between the popularity of the left-leaning Truther theory and the right-leaning Birther theory, despite each side’s wishes to the contrary.

But Kennedy conspiracy theories are significantly more popular. Polls find that between 60 and 80 percent of Americans reject the idea that Kennedy was killed by Lee Harvey Oswald acting alone. In fact, more Americans believe that a shadowy conspiracy was behind a president’s death 50 years ago than know who Joe Biden is. Why are Kennedy assassination conspiracy theories so popular? The distinguishing feature of a successful conspiracy theory is power, and the Kennedy assassination has that in spades. The victim was an American president and the potential villains include actors of immense reach and influence. There are so many accused conspirators that anyone, regardless of political affiliation, can find a detested powerful actor to blame. For those on the right there is Lyndon Johnson, Fidel Castro and the Soviet Union; for the left there is Lyndon Johnson, defense contractors and the military. And this is only a partial list.

Conspiracy theories are not only about power but proof. High-quality information does change minds: When the Watergate story first broke, many dismissed the charges as partisan. Because high-quality evidence became available — hearings were held, evidence was presented, co-conspirators admitted to their crimes — virtually everyone now believes that Nixon conspired to commit and cover up crimes. But unlike the Birther and Truther theories, which languish for lack of impartial support, JFK conspiracy theories have some impartial support.

Calling it “proof” might be generous, but Kennedy assassination conspiracy theories clear higher evidentiary bars. There is evidence that the government hurried the investigation of the president’s murder and was not eager to find high-profile scapegoats. Most accounts attribute that rush to a pragmatic desire not to wrongfully implicate the Soviets and dangerously destabilize superpower relations. Most notably an official congressional inquiry seemed convinced of a conspiracy, so even those with moderate conspiratorial predispositions have something to hang their hat on. Because of their evolving scapegoats and above-average amount of evidence, JFK conspiracy theories have lodged themselves into the collective consciousness.

This is unlikely to happen again in the short term. Our analyses suggest the overall level of conspiracy theorizing in the United States has been going down steadily since Kennedy’s assassination. The U.S. government looks too ungainly to control and too leaky to seal. Liberals and conservatives increasingly see the world differently and now debate conspiracy theories about different events rather than different interpretations of the same event. While often a scapegoat, the Internet does not appear to have changed conspiracy theorizing any more than television or radio did. Our research shows that the vast majority of Internet news discusses conspiracy theories mostly to mock or debunk them. Of the 100 or so most trafficked Web sites in the United States, none are primarily focused on conspiracy theories.

While current conditions suggest that Kennedy assassination conspiracy theories will remain prominent for some time, they will not do so forever. With titanic theories about Freemasons, Abolitionists, robber barons, communists and more in our past, something new and big will eventually come along when the conditions are more amenable. Whether it will rank with the most momentous in American history is worth debating. Ever hear the one about King George III’s plans to enslave North America? Now there was a conspiracy theory.

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Comments our editors find particularly useful or relevant are displayed in Top Comments, as are comments by users with these badges: . Replies to those posts appear here, as well as posts by staff writers.